Absorbent articles represented by disposable diapers or the like have a structure in which an absorbent material for absorbing a liquid such as a body liquid is sandwiched with a flexible liquid-permeable surface sheet (top sheet) positioned on a side contacting a body and a liquid-impermeable backside sheet (back sheet) positioned on a side opposite to that contacting the body.
Conventionally, there have been increasing demands for thinning and light-weighing of absorbent articles, from the viewpoint of designing property, convenience upon carrying, and efficiency upon distribution. Further, in the recent years, there have been growing needs for so-called eco-friendly intentions, in which resources are effectively utilized so that use of natural materials that require a long time to grow such as trees is avoided as much as possible, from the viewpoint of environmental protection. Conventionally, a method for thinning that is generally carried out in absorbent articles is, for example, a method of reducing hydrophilic fibers such as crushed pulp of a wood material, which has a role of fixing a water-absorbent resin in an absorbent material, while increasing a water-absorbent resin.
An absorbent material in which a water-absorbent resin having a smaller volume and higher water-absorbent capacity is used in a large amount with a lowered proportion of a hydrophilic fiber being bulky and having lower water-absorbent properties is intended to achieve thinning by reducing bulky materials while obtaining absorption capacity matching the design of an absorbent article, so that it is considered as a reasonable improved method. However, when distribution or diffusion of a liquid upon actually using in an absorbent article such as disposable diapers is considered, there is a disadvantage that if a large amount of the water-absorbent resin is formed into a soft gel-like state by absorption of the liquid, a so-called “gel-blocking phenomenon” takes place, whereby liquid diffusibility is markedly lowered and a liquid permeation rate of the absorbent material is slowed down. This “gel-blocking phenomenon” is a phenomenon in which especially when an absorbent material in which water-absorbent resins are highly densified absorbs a liquid, water-absorbent resins existing near a surface layer absorb the liquid to form soft gels that are even more densified near the surface layer, so that a liquid permeation into an internal of an absorbent material is blocked, thereby making the internal of the water-absorbent resin incapable of efficiently absorbing the liquid.
In view of the above, conventionally, as a means of inhibiting gel-blocking phenomenon which takes place by reducing hydrophilic fibers while using a water-absorbent resin in a large amount, for example, proposals such as a method using an absorbent polymer having such properties as specified Saline Flow Conductivity and Performance under Pressure (see Patent Publication 1), and a method using a water-absorbent resin prepared by heat-treating a specified water-absorbent resin precursor with a specified surface crosslinking agent (see Patent Publication 2) have been made.
However, in these methods, the liquid absorbent properties as absorbent materials in which water-absorbent resins are used in large amounts are not satisfactory. In addition, there arise some problems that the water-absorbent resin is subjected to be mobile before use or during use because hydrophilic fibers that play a role of fixing the water-absorbent resin are reduced. The absorbent material in which the localization of the absorbent resin takes place is more likely to cause gel-blocking phenomenon.
Further, an absorbent material of which hydrophilic fibers that contribute to retention of the form are reduced has a lowered shape-retaining ability as an absorbent material, so that deformation in shapes such as twist-bending or tear before or after the absorption of a liquid is likely to take place. An absorbent material with deformation in shapes has markedly lowered liquid diffusibility, so that abilities inherently owned by the absorbent material cannot be exhibited. In order to try to avoid such phenomena, a ratio of hydrophilic fibers and a water-absorbent resin would be limited, thereby posing limitations in the thinning of an absorbent article.
In view of the above, in recent years, as a next generation style absorbent material which is capable of increasing a content of a water-absorbent resin while using hydrophilic fibers in an absorbent material as little as possible, studies have been widely made on an absorbent laminate that substantially does not contain hydrophilic fibers in an absorbent layer, a water-absorbent sheet or the like. The studies include, for example, a method of keeping a water-absorbent resin in reticulation of a bulky nonwoven fabric (see Patent Publication 3), a method of sealing a water-absorbent polymer between two sheets of meltblown nonwoven fabrics (see Patent Publication 4), a method of interposing water-absorbent polymer particles between a hydrophobic nonwoven fabric and a hydrophilic sheet (see Patent Publication 5), and the like.
However, in a case where hydrophilic fibers are hardly used, the gel-blocking phenomenon as mentioned above is likely to take place. Even in a case where gel-blocking phenomenon does not take place, a thing that would serve the role of conventional hydrophilic fibers by which a body fluid such as urine is temporarily subjected to water retention and diffusion of the liquid to an overall absorbent material is lacking, so that a liquid leakage is likely to occur in the absorbent laminate, without being able to sufficiently capture the liquid.
Further, when an adhesive is used for retaining the shape of an absorbent laminate, the surface of a water-absorbent resin is coated with an adhesive, so that liquid absorbent properties are likely to be lowered. Alternatively, an upper side and a lower side of nonwoven fabrics are firmly adhered with an adhesive to confine an water-absorbent resin in a pouched form or the like, so that the water-absorbent properties inherently owned by the water-absorbent resin are less likely to be exhibited.
When adhesive strength of an absorbent laminate is weakened in order to improve liquid absorbent properties of the above-mentioned absorbent laminate, not only a large amount of the absorbent resin is detached upon working on the laminate, thereby making unfavorable economically, but also the laminate is exfoliated due to deficiency in adhesive strength, so that there are some possibilities of loss of commercial values. In other words, if the adhesion is strengthened, the gel-blocking phenomenon or liquid leakage occurs, and if the adhesion is weakened, it would lead to the detachment of a water-absorbent resin and the breaking of the laminate, so that an absorbent laminate or a water-absorbent sheet for which the above-mentioned problems are solved is not yet obtained at present.
Studies on improvement of the balance between adhesion and the liquid absorbent properties in the water-absorbent sheets as described above are also made. The studies include, for example, a method of using an absorbent laminate comprising two sheets of nonwoven fabrics adhered with reticular layers provided between the nonwoven fabrics, comprising upper and lower two layers of hot melt adhesives (see Patent Publication 6), a method of applying a specified reactive hot melt to a substrate made of a nonwoven fabric or a film, thereby fixing a water-absorbent resin (see Patent Publication 7), a method of coating a fine cellulose and a water-absorbent resin with a network-like hot melt to hold them (see Patent Publication 8), and the like. However, even if properties of a nonwoven fabric, a water-absorbent resin, and an adhesive, or conditions of use thereof are defined, it is difficult to obtain a water-absorbent sheet having high liquid absorbent properties and shape retaining ability. In addition, if a specified adhesive or method of adhesion is used, it is not desirable from the viewpoint of economical advantages and productivity, even if the liquid absorbent properties were improved.
There is also a method of immobilizing a water-absorbent resin to a substrate without using an adhesive. The method includes, for example, a method of adhering water-absorbent polymer particles in the process of polymerization to a synthetic fibrous substrate to carry out polymerization on the fibrous substrate (see Patent Publication 9), a method of polymerizing a monomer aqueous composition containing acrylic acid and an acrylic acid salt as main components on a nonwoven fabric substrate by means of electron beam irradiation (see Patent Publication 10), and the like.
In these methods, while the synthetic fibrous substrate is penetrated into the polymer particles to be firmly adhered, there are some disadvantages that it is difficult to complete the polymerization reaction in the substrate, so that unreacted monomers and the like remain in the substrate in large amounts.